My best GUESS, given the mortality rate in that era is that males who fought all the wars whatever his social class, would be anytime after 16 and before 25. If they were allowed by law to marry at 16, then it would follow that they were considered "adult" (whether they could afford to marry is an entirely different question)
If in the english and german nations at least the child was put to an apprenticeship at 14 and for -- usually -- a 7-yr term, then one supposes the age of majority for males at least would be 21?
The Laws of many lands don't seem to have changed all that much as to when a child can be considered legally competent to govern its own life and to be responsible legally for their own acts and liable for any defaults.
Anyone looking around would suppose that no child is "competent" until nearly 26 or so when they seem to "sober up"
HOWEVER, there is your local friendly reference desk librarian and it is possible this person with degrees in Librarianship would have some suggestions at least on where you could look and possibly arrange a loan if they don't have on site the materials ...
If there's a local college that's a good place to start questioning history instructors (european) and the head librarian
I've run out of ideas here